While chromatin provides a means for packaging large amounts of DNA inside a small volume, it is also the substrate for key processes such as replication, recombination and transcription. It has become increasingly apparent during past decade that eukaryotic cells often control these processes by mechanisms that depend upon chromatin structure. Genes can be activated or silenced by factors that changes the position of nucleosomes around enhancers and promoter. Chromatin modifications may also influence gene expression. For example, hyperactivation of X-linked gene expression in male flies is accompanied by an increase in histone H4 acetylation, while silencing of the mating type loci in yeast is accompanied by deacetylation The research described in this proposal focuses on regulatory elements that appear to exert their effects by chromatin dependent mechanisms. Two different class of regulatory elements will be examined. The first are domain boundary elements or insulators, while the second are Polycomb dependent silencers or PREs.